Lots of words that need to be defined here. First off I shall take on enzyme. Yes, it is a protein (made up of amino acids in a specific configuration) that has a specific function in a reaction that it will help the reaction proceed (catalyst).

Now lets take on "membrane-bound". Membrane-bound means that the enzyme/catalytic protein is embeded in the membrane of the enclosed organelle of the cell it belongs to. I found a really nice example of membrane-bound enzymes, whose function is photosynthesis:http://en.wikipedia.org/wiki/File:Thyla ... mbrane.png(Membrane-bound enzymes can also be like the ferrodoxin and the plastocyanin, in which the enzymes are membrane-bound to only one one side of the whole membrane, or even like plastoquinone in which it is inside the membrane).

And last is "receptor". The enzyme can have its "active" site on the outside of the cell that acts like a receptor for a molecular substrate (usually called ligand) to bind to it. This will cause a reaction in the enzyme, so the bound ligand distorts the enzyme in such a way that a new 'active' site happens at the other end of the enzyme (thru the membrane), and this produces a new reaction to the inside of the cell (such as phosphorylation or proton (H+) pumping).

The whole sentence as you stated in the beginning is correct: a protein can serve as a membrane-bound receptor molecule.

Ligand is the molecule that the protein (membrane-bound receptor) receives in it's ' binding site (or a binding domain)'. Ligands could be a protein molecule, but it can also be other molecules:http://en.wikipedia.org/wiki/Receptor_proteins

Receptor means it 'receives' something. (Baseball glove)Ligand is the something. (Baseball)Membrane-bound means the location: the protein is located in the membrane.(The membrane could be of the cell/plasma membrane, or the nuclear membrane/nucleus, or the mitochondrion's outside membrane or the inner membrane, the endoplasmic reticulum, or of the any other membrane-enclosed organelles of a cell).

If you read the wikipedia entry on receptor proteins, you will see that a receptor can be bound to a membrane, but it also can just be floating around in the cytoplasm (waiting to bind to its ligand, when it becomes available).

twinklestar1012 wrote: "protein can act as a membrane-bound receptor molecule"

Yes. Enzyme is a way of 'acting' or 'serving'. It is what the protein can do. It is the protein's function.So you have 'protein' - composed of amino acids in a specific configuration'membrane-bound' - its location/where it is/what it is attached to'receptor' - part of the function, just a specific part of being the enzyme's function.

Remember that proteins can be enzymes, but they are not only enzymes. Proteins can be structural as well as functional.

i can understand the meaning of "protein can serve as membrane-bound receptor molecule (e.g. in photosynthesis)"but not "protein can serve as membrane-bound enzyme receptor molecule (e.g. in photosynthesis)"does it actually mean 2 *separate* things:1. protein can serve as membrane-bound receptor molecule (e.g. in photosynthesis)2. protein can serve as enzyme

or it means a kind of protein can serve as membrane-bound receptor molecule, at the same time, it is an enzyme. (2 functions for one protein)?

A protein, bound to the membrane, is able to act as a receptor molecule to bind to a mobile enzyme (in the case of photosynthesis, this is probably referring to a membrane-soluble electron carrier such as ferredoxin or flavidoxin).